Method for treating recycled polyethylene terephthalate

ABSTRACT

A process for treating RPET flakes comprises providing a quantity of RPET flakes, comminuting the RPET flakes to prepare RPET particles having an average mean particle size less than about 300 microns, and treating the RPET particles utilizing a low energy process selected from the group consisting of simultaneously melting and mixing the RPET particles by means of a low energy melting device to prepare an RPET melt, and thermally treating the RPET particles to dry or crystallize the RPET particles.

CROSS REFERENCE TO RELATED APPLICATION

This Application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/459,904 filed on Apr. 2, 2003.

FIELD OF THE INVENTION

This invention relates generally to an improved method for treatingrecycled polyethylene terephthalate (RPET). More particularly, theinvention is directed to a method for treating RPET so that it can bemelted utilizing low energy processing equipment or thermally treatedmore quickly with less energy.

BACKGROUND OF THE INVENTION

Post-consumer processing of recycled PET to manufacture a variety oflow-tech consumer products such as flower pots and fence posts iswell-known. Typically, the recycling process utilizes used PETcontainers, such as discarded carbonated beverage containers, which arecollected, sorted, washed, and separated from contaminants to yield arelatively clean source of RPET. Additionally, the manufacture ofimperfect and damaged molded PET products, particularly the blow moldedbottles used for containing consumer goods, results in a considerableamount of PET waste which the manufacturers of such products would liketo reuse. The RPET produced by conventional recycling processes isgenerally in ground or flake form, which is thereafter melt processed orfurther pelletized by the end user.

RPET is generally subjected to a grinding operation in order to make thematerial easier to handle and process. Conventional grinding equipmentreduces the RPET to about ⅜ inch particles or flakes. The grinding isconducted in a manner to insure that a consistent flake size will beproduced, by employing a grate or screen through which the groundmaterial must pass upon exiting the grinder. Although conventional RPETflakes melt processing and pelletizing equipment is designed to handle ⅜inch flakes, some RPET materials having sizes as large as ½ inch and assmall as ¼ inch are also commercially produced. The bulk density of ⅜inch flake RPET generally ranges from about 22 to about 35 pounds percubic foot.

Considerable post-processing of RPET is typically necessary for themanufacture of, for example, plastic containers. Many process systemsand procedures have been devised to treat ⅜ inch RPET flakes. Morespecifically, the vast majority of extruders and extruder barrels andscrews have been designed to feed, melt, mix, and meter ⅜ inch RPETflake feed stock. Other heat treating equipment such as, for example,crystallizers and dryers have likewise been designed to accept ⅜ inchRPET flakes. The operation of these devices, however, requires a greatamount of energy and long processing times, to thermally treat, orprepare a polymer melt from, the industry standard ⅜ inch RPET flakes.

It would be desirable to thermally treat, or prepare a polymer meltfrom, recycled polyethylene terephthalate, utilizing low energy thermalprocessing and melting equipment.

SUMMARY OF THE INVENTION

Accordant with the present invention, an improved process for thermallytreating or preparing a melt from RPET, utilizing low energy processingequipment, has surprisingly been discovered. The process comprisesproviding a quantity of RPET flakes, comminuting the RPET flakes toprepare RPET particles having an average mean particle size less thanabout 300 microns, and treating the RPET particles utilizing a lowenergy process selected from the group consisting of simultaneouslymelting and mixing the RPET particles by means of a low energy meltingdevice to prepare an RPET melt, and thermally treating the RPETparticles to dry or crystallize the RPET particles.

The inventive process is particularly useful for treating RPET flakesfor the subsequent processing and forming of, for example, plasticcontainers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed to a process for treating RPET flakesutilizing low energy processing equipment, comprising providing aquantity of RPET flakes, comminuting the RPET flakes to prepare. RPETparticles having an average mean particle size less than about 300microns, and treating the RPET particles utilizing a low energy processselected from the group consisting of simultaneously melting and mixingthe RPET particles by means of a low energy melting device to prepare anRPET melt, and thermally treating the RPET particles to dry orcrystallize the RPET particles.

By the term “RPET flakes” as it is used herein is meant generally thecommercially available recycled polyethylene terephthalate materialsproduced by conventional polyethylene terephthalate recycling methods,usually in flake form, but which may additionally be in the form ofchunks, spheres, pellets, and the like, and which are generally madeavailable in bulk in a substantially uniform particle size from about ¼inch to about ½ inch.

According to the present invention, a quantity of RPET flakes isprovided for further processing. The quantity of RPET flakes provided inthe initial step of the inventive process may easily be determined by aroutineer in the art of polymer processing, depending upon the quantityof RPET ultimately desired.

According to the present invention, the RPET flakes are comminuted byany conventional means, to prepare RPET particles having an average meanparticle size less than about 300 microns. Methods and apparatus forcomminuting RPET and other polymers are well known in the art.

Following comminuting of the RPET flakes, the resultant RPET particlesare treated, utilizing a low energy process. The low energy process maycomprise either the preparation of an RPET melt or the thermalprocessing of the RPET particles to effect their drying andcrystallization.

In the first alternative treatment step, the RPET particles aresimultaneously melted and mixed utilizing conventional low energyequipment such as, for example, a 2-roll mill, a heated casting roll, arotating mandrel to effect frictional melting, or the like. Theresultant RPET polymer melt is beneficial for combining with otherpolymer materials for subsequent melt processing and forming operations.For example, the addition of the inventive RPET melt to a quantity ofvirgin polyethylene terephthalate (PET) will extend the volume of thevirgin PET.

By contrast to the present invention, RPET flakes have formerly beenprocessed utilizing large extruders having complicated screw designs tohandle the ⅜ inch flake starting material. The present inventive processbenefits from the much greater surface-to-volume ratio of the RPETparticles (relative to the RPET flakes) to facilitate the melting andthorough mixing using low energy shear devices.

In the second alternative treatment step, the RPET particles arethermally treated to dry and/or crystallize same. This may be achievedby conventional means such as, for example, heating the mass of RPETparticles to a temperature below the melt temperature of polyethyleneterephthalate. This heating may be accomplished, for example, by passinga gas over or through the bed of RPET particles. Examples of usefulgases include, but are not limited to, air, nitrogen, argon, and thelike, as well as mixtures thereof.

By contrast to the present invention, RPET flakes have formerly beendried and/or crystallized in heated chambers for required periods oftime up to about five hours. The present inventive process againbenefits from the large surface-to-volume ratio of the RPET particles,to achieve the same levels of dryness and/or crystallinity inapproximately fifteen minutes; thus requiring less energy and time toaccomplish the same result.

The process for treating RPET flakes utilizing low energy processingequipment described hereinabove is generally disclosed in terms of itsbroadest application to the practice of the present inventionoccasionally, the process conditions as described may not be preciselyapplicable to each RPET flake and low energy process combinationincluded within the disclosed scope. Those instances where this occurs,however, will be readily recognized by those ordinarily skilled in theart. In all such cases, the process may be successfully performed byconventional modifications to the disclosed method.

The invention is more easily comprehended by reference to specificembodiments recited hereinabove which are representative of theinvention. It must be understood, however, that the specific embodimentsare provided only for the purpose of illustration, and that theinvention may be practiced otherwise than as specifically illustratedwithout departing from its spirit and scope.

1. A process for treating RPET flakes, comprising: providing a quantityof RPET flakes; comminuting the RPET flakes, to prepare RPET particleshaving an average mean particle size less than about 300 microns; andtreating the RPET particles utilizing a low energy process selected fromthe group consisting of simultaneously melting and mixing the RPETparticles by means of a low energy melting device to prepare an RPETmelt, and thermally treating the RPET particles to dry or crystallizethe RPET particles.
 2. The process for treating RPET flakes according toclaim 1, wherein the RPET flakes comprise chunks, spheres, pellets, ormixtures thereof.
 3. The process for treating RPET flakes according toclaim 1, wherein the RPET flakes have particle sizes from about ¼ inchto about ½ inch.
 4. The process for treating RPET flakes according toclaim 1, wherein the simultaneous melting and mixing step isaccomplished using a low energy melting device selected from the groupconsisting of a 2-roll mill, a heated casting roll, and a rotatingmandrel.
 5. The process for treating RPET flakes according to claim 1,wherein the thermal treating step is accomplished by heating the RPETparticles to a temperature below the melt temperature of polyethyleneterephthalate.
 6. The process for treating RPET flakes according toclaim 5, wherein the RPET particles are heated by passing a gas over orthrough the bed of RPET particles.
 7. The process for treating RPETflakes according to claim 6, wherein the gas comprises air, nitrogen,argon, or mixtures thereof.
 8. A process for treating RPET flakes,comprising: providing a quantity of RPET flakes, comprising chunks,spheres, pellets, or mixtures thereof, having particle sizes from about¼ inch to about ½ inch; comminuting the RPET flakes, to prepare RPETparticles having an average mean particle size less than about 300microns; and treating the RPET particles utilizing a low energy processselected from the group consisting of simultaneously melting and mixingthe RPET particles by means of a low energy melting device selected fromthe group consisting of a 2-roll mill, a heated casting roll, and arotating mandrel, to prepare an RPET melt, and thermally treating theRPET particles by heating the RPET particles to a temperature below themelt temperature of polyethylene terephthalate by passing a gascomprising air, nitrogen, argon, or mixtures thereof over or through thebed of RPET particles, to dry or crystallize the RPET particles.